I. Field
This disclosure relates to methods and apparatus for creating and controlling transient cavitation in a liquid.
II. Description of Related Art
Cavitation is generally known and defined as the activity of bubbles (e.g., gas bubbles) in a liquid. Such activity includes growth, pulsation and/or collapse of bubbles in a liquid. The pulsation of bubbles is known as stable cavitation, whereas the collapse of bubbles is known as transient cavitation. The occurrence of transient cavitation can release high amounts of energy towards an area surrounding the cavitation. Such energy may be, for example, in the form of heat, shockwaves, etc.
Transient cavitation is applied in a large number of technical fields. For example, in sonochemistry, bubbles collapsing in an ultrasonic field have a catalytic effect on chemical reactions. Also, cavitation is used in medical applications, for example, as a contrast enhancer in ultrasound diagnostics. However, one of the best-known applications of cavitation may be the removal of particles from a surface of a substrate, such as a semiconductor substrate.
A common problem in these various applications is control of transient cavitation such that it occurs in a desired fashion with respect to location and mechanics of bubble collapses. For example, in substrate cleaning technology, a common problem is non-uniform removal of particles from substrate surfaces. In such applications, it is desirable that the particles being removed are removed from the substrate surface without damaging the substrate as a result of “heavy collapse” and that a uniformly cleaned surface results. This outcome is difficult to accomplish using current approaches.
Particle removal mechanisms have been previously studied for approaches that include (i) gasifying a cleaning liquid with gasses such as oxygen, nitrogen, argon, xenon, carbon dioxide, etc., or (ii) degassing a cleaning liquid. These liquids are then used to clean a wafer and sonoluminescence (SL) signals are generated by collapsing bubbles. Furthermore, after the cleaning process, particle removal efficiency is determined.
FIG. 1 is graph which demonstrates that the presence of gas in a liquid (a gasified liquid) results in achieving higher particle removal efficiency (PRE) percentages. FIG. 2 is a graph that illustrates SL signals that are plotted for gasified and degassed liquids. In gasified liquids, SL signals, generated by collapsing bubbles, can be detected as compared to a degassed liquid in which little to no SL signal is detected. The combination of FIGS. 1 and 2 illustrates that transient cavitation is one basis for particle removal in such approaches.
FIG. 3 is a graph presented by Neppiras in Acoustic Cavitation, 1980, which illustrates, that dependent on a driving pressure, a frequency of an acoustic field and gas bubble radius, gas bubbles can grow (zone Y) and then collapse when entering area Z, or may dissolve without collapse, if the bubbles are small enough to enter zone X.
In U.S. Pat. No. 6,048,405 to Skrovan (hereafter “Skrovan”), a cleaning method for use in the microelectronics industry is disclosed. In the method described in Slcrovan, gas bubbles are introduced below the surface of a substrate assembly such that the bubbles pass across the surface as they rise in the liquid. In the method described in Skrovan, the substrate assembly is immersed in a region or a liquid through which megasonic energy is projected. In Skrovan, particles are released from the surface by shoclwaves from the megasonic field.
As was demonstrated by FIGS. 1 and 2, the use of megasonic energy alone is not an efficient approach for particle removal as compared to gasified liquid in combination with megasonic energy. Therefore, application of the method disclosed in Skrovan results in a non-uniform cleaning being obtained. Further, simply gasifying the liquid used in the method of Skrovan could result in substrate damage, e.g., due to heavy collapse. Thus, alternative approaches for cleaning processes using gasified liquids are desirable.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.